1. Technical Field
This invention generally relates to rigid or hard disk drives or tape recording devices which store information or data on rotating or moving storage media. More specifically, the invention relates to a method and apparatus for dynamically controlling the flying behavior of a read/write magnetic head as a rotating magnetic disk spins under the head or as a moving magnetic tape moves under the head.
2. Description of the Related Art
Magnetic disk storage devices known in the prior art comprise an air-bearing slider. The slider itself is mounted on a so-called suspension arm which essentially functions as a load beam and which comprises a rigid portion and an elastic portion, wherein the elastic portion generates a load for urging the slider towards or away of the recording medium, so that the slider on which an electromagnetic transducer is mounted is pushed to the recording medium or pulled away from the recording medium with a certain necessary load.
For example, when a recording medium such as a magnetic recording disk is rotating, the load exerted on the slider is balanced by the airflow of an air cushion force exerted on the air bearing surface of the slider. As a result, the slider flies above the recording medium with a very fine gap, the so-called xe2x80x9cflying height.xe2x80x9d
The flying height of a slider carrying a magnetic read/write head above a rotating magnetic hard disk lies currently in the range of 20 nanometers and will likely decrease in the future. In currently known hard disk devices the flying height is substantially determined by the particular design of an air bearing surface of the slider and a head gimbal assembly. But thereupon variations of the flying height within a few nanometers may exist for non-identical devices of the same type owing to fabrication tolerances. These tolerances considerably influence the writing and reading characteristics to and from the hard disk.
Two different groups of types of prior art magnetic head suspension mechanisms are known. In a first group, a thin film formed on the elastic portion of the suspension arm which is expandable or shrinkable due to an internal stress of the film itself causes the elastic portion and thereby the arm to bend thus varying the flying height of the read/write head above the disk surface. However, the bending of the suspension arm towards the disk surface increases the spring force pushing against the aerodynamic lift of the slider.
Transducers for bending the suspension arm, piezoelectric films, or bi-metallic strips have been proposed in the art. Approaches belonging to the first mentioned group are disclosed for instance in U.S. Pat. No. 5,825,590, xe2x80x9cMagnetic Head Suspension Mechanism with a Thin Film Thereon For Creating a Bent Portion of a Vibration Absorbing Portion,xe2x80x9d assigned to Fujitsu Ltd., Japan and in U.S. Pat. No. 5,377,058, xe2x80x9cFly Height Servo Control of Read/Write Head Suspensionxe2x80x9d assigned to IBM Corporation, New York.
Another group of prior art approaches provides for a one-time and static adjustment of the spring pressure and/or the shape of the bending of the suspension arm. In Japanese Abstract JP 5189906 A, during production of floating type magnetic heads, it is therein proposed to set the floating amount of a slider at a small and exact amount by transferring heat energy to the spring part of the suspension arm and adjusting the spring pressure. Applying the heat is particularly accomplished by irradiating the spring with a laser beam and setting the bending amount in the rolling direction of the spring part by a discrete spring pressure regulation. After having set the bending amount the slider is mounted on the suspension arm.
According to another static approach disclosed in Japanese Abstract JP 63281283 A, the spring part is made of a superelastic material such as a shape-memory alloy which allows that the initial form of the spring part is rather a bent shape. The initial form is deformed until it is approximately parallel to the disk surface. Since only the spring part is made of the super elastic material, an in-plane rigidity is ensured and in addition the spring constant is minimized.
The above approaches using a transducer element have the drawback that such an element requires larger areas on the suspension what, for instance, complicates further miniaturization of the suspension arm.
The concept underlying the present invention is to influence or to vary the spring constant of a suspension and therefore to influence the equilibrium state of the prementioned two opposite forces, one resulting from the aircushion underneath the air-bearing surface of the slider thus causing a lift or sink force and the other force provided by the spring force of the suspension itself. By disturbing this equilibrium state of forces, the flying height of the read/write head can be changed in a controlled manner. By a controlled variation of the spring constant, the flying height therefore can be adjusted very precisely.
In contrast to the prior art approaches described above, the present invention provides for a passive as well as a active mechanism where actuation of the read-/write head is accomplished only by varying the spring constant k, and not by active deflection DX as in the prior art approaches, F=kxe2x88x92DX. In contrast to the prior art approaches, where the suspension is deflected actively by providing a bending force or bending momentum, according to the present invention the suspension, moreover, is being bent merely as a consequence of the new equilibrium of the mechanical forces acting upon the slider. In addition or alternative to the bending, the suspension can also be bent locally or even buckled by locally varying the spring constant give by the elastic modulus of the suspension material, respectively.
This invention therefore enables adjusting of an incorrect gramload or varying gramload only by (actively) adapting the spring constant appropriately. This can be accomplished already during the manufacturing process for the disk storage device after the suspension together with the read/write head have been assembled to the storage device or it can be accomplished during HDD operation as an active adjustment tool.
In addition, the proposed actuation scheme can also be implemented in a very small area on the suspension thus, for instance, enabling further miniaturization of the suspension arm.
For measuring the flying height of the head, there exist a number of methods and systems e.g. a thermal proximity sensing technique used in scanning probe microscopy sensors and exemplarily disclosed in U.S. Pat. No. 5,527,110 (Abraham et al.) entitled, xe2x80x9cMethod and Apparatus for Detecting Asperities on Magnetic Disks using Thermal Proximity Imagingxe2x80x9d. As the spinning velocity of the hard disk is nearly constant during operation of the disk storage device, the lift or sink forces are nearly constant too for a given design of the slider, in particular its shape of the air-bearing surface, thus allowing a one-time adjustment of an incorrect gramload. In addition, a particular flying height can be set during the manufacturing process.
Besides a static adjustment of the spring force by way of varying the spring constant, also an active or dynamic flying height adjustment can be accomplished to guarantee a good read and write characteristic and particularly the operation characteristic of the underlying spinning storage system.
Thereupon, through the high-speed spinning of the disk, the entire disk storage device will normally change its temperature during operation. This does also raise the temperature of the suspension arm and the air temperature inside a disk storage housing. Different temperatures of the suspension arm particularly cause changes of the spring force and thus corresponding changes of the flying height of the head. Variations of the air pressure or contamination of the air bearing surface have an impact on the air streaming underneath the slider and thus influence the lift force actuating on the slider. All these effects influencing the flying height can be dynamically adjusted by the present invention. Thereupon, variations in the speed of rotation of the hard disk and its influence on the flying height of the read/write head can be adjusted accordingly, for example in a low and high performance mode of the HDD.
Further environmental properties can be the degradation of materials over the time of operation of the hard disk which therefore has to be corrected to keep the optimum flying height in order to achieve reliability of the hard disk.
The invention also allows for dynamically setting a particular flying height so that the read/write head can be kept at a minimum flying height only during the read/write head is reading or writing to the magnetic disk. When there is no reading or writing access, which is about 90% of the hard disk spinning time, the read/write head can be withdrawn from the minimum flying height in order to enhance mechanical robustness of the hard disk. Thus, the invention enables dynamic two- or more-level operation of the read/write head.
Another technical field for advantageously applying the invention is future read and write processes which might need for both a different but defined flying height in order to achieve an ideal performance such as the storage density. For example, it might be more convenient to write magnetic bits at very small flying height while reading of bits is still possible at a higher flying height. In particular during the seek the servo pattern can be read with sufficient resolution at higher flying height. This would result in a decreased seek time. By actively controlling the flying height this could be realized and would also increase the life-time of the hard disk.
Changing the flying height thereupon enables to position the read/write head in a kind of parking position, where it is protected from damage thus being shock resistive. Additionally, when the slider is far away from the disk surface, the air friction force between the slider and the disk surface decreases and thus decreases, as a consequence, the power consumption which is needed to spin the hard disk.
Still another advantageous application field of the invention is use of disk storage devices in portable devices like laptops, mobile telephones or the like, where it might be useful to reduce power consumption by reducing the spinning velocity of the hard disk. Reducing the velocity of rotation of the disk may have an effect on the air-bearing forces and therefore might result in a variation of the flying height thus changing the performance of the device.
The variable suspension spring constant, in a first embodiment, is realized by varying locally the material temperature of the suspension arm. Such a temperature variation can be realized by a small heater deposited onto the suspension arm, for instance a thin or thick film structure deposited on the surface of the suspension arm. Varying the elastic properties of such a layer, which especially can change the spring constant in dependence of temperature, causes a variation of the bending of the suspension arm and therefore a variation of the flying height of the read/write head.
According to another embodiment, the variation of elastic properties can also be realized by using a bulk material or layer for the suspension arm or an according additional layer with a magnetoelastic effect where the spring constant of the material depends on the magnetization. The magnetization can be controlled by an external as well as by an internal applied magnetic field acting upon the magnetosensitive layer. An internal magnetic field source can be realized by a second e.g. hard magnetic layer deposited beneath or on top or close to the magnetosensitive material or by adding a small loop as inductive element to generate a magnetic field. Such a magnetic layer can be also used to bias or linearize the effect. Using a magnetoelastic layer or material has the advantage that there is not needed a continuous power consumption in the storage device since the magnetoelastic effect is a quasi-static effect.
It is noteworthy that the described concepts can be carried out individually for each slider/hard disk combination and do not depend on the peculiarities of the underlying disk storage device. Beyond that the concepts are not limited to magnetic hard disk storage devices and can also advantageously be applied to other present and future spinning disks based storage devices like optical disks etc.
Accordingly, it is an object of the invention to provide a method and apparatus for controlling the flying height of a read/write head above a moving storage medium, like a rotating magnetic hard disk or a tape of a tape recording device, which allow for an active and/or dynamic flying height adjustment or, particularly in case of a tape recording device, adjustment of the bearing pressure of its read/write head.
It is another object to provide a method and apparatus to allow for precisely setting the flying height of individual storage devices, in particular hard disk drives, at a defined value in order to meet fabrication tolerances.
It is another object to provide a method and apparatus which allow to compensate pressure changes in a hard disk drive and/or contamination of an air bearing surface in a hard disk drive or a tape storage device.
It is yet another object of the invention to provide a method and apparatus for controlling roll, pitch and twist of a read/write head above a rotating magnetic hard disk or above a moving tape of a tape store which allow for an active alignment or adjustment of these properties.